1. Field of the Invention
The present invention relates to a wideband (DC to GHz) printed circuit board balun. This wideband balun is suitable for use in communication systems, radars, radio frequency transmitters, receivers, signal processors, and more specifically to ultra wide band (UWB) applications such as impulse radar. The word "balun" was derived by joining the words "balanced" and "unbalanced" together, since it is the function of a balun to connect a balanced electrical network to an unbalanced electrical network. A balanced network is one where no ground currents flow and instead, there is a "plus" wire and a "minus" wire that counteract or balance each other.
2. Description of the Prior Art
A great variety of baluns are available commercially that cover a broad spectrum in terms of size, bandwidth, center frequency, and insertion loss. However, these commercial baluns do not have the wide bandwidth, balance, insertion loss, or power handling capability required for ultra wide bandwidth applications such as impulse radar. Impulse radar is presently being used in a variety of radar systems to detect aircraft, ground vehicles, people, mines, buried pipes, roadway faults, buried homicide victims, tunnels, leaking buried pipes, and similar items. Consequently, it is desirable to have a balun that maintains low insertion loss and good balance for UWB applications. Accordingly, it is an object of this invention to provide a balun that maintains low insertion loss and good balance for UWB applications.
A recent example of a wide bandwidth balun is shown in U.S. Pat. No. 5,523,728 by the same inventor, filed on Aug. 17, 1994. U.S. Pat. No. 5,523,728 uses transmission lines formed by microstrip conductors on a printed circuit board to achieve a wide bandwidth. The balanced transmission lines formed by the balun are stacked about a ground plane which causes the fields of the signals to be aligned. In addition, the balun can divide the signal into more than two microstrip transmission lines, combine them, and stack balanced signal line outputs in order to increase the impedance ratio of the balun. Ferrite cores placed about the transmission lines and resistor-capacitor circuits improve the low frequency performance of the balun. When ferrite cores are placed about the transmission lines, however, there is a large depreciation in signal strength at high frequencies, limiting the frequency range of the balun.